Thin ribbons of specially patterned metal foil, called carrier tapes, are used in the semiconductor industry to handle very small and fragile components such as integrated circuit chips. The chips are bonded to the tape which moves and carries the chips until they are later incorporated into subassemblies. The tape also provides leads to bondably connect the chips to the subassemblies.
Carrier tapes typically include at least a conductive material such as copper sheet out of which the leads are formed. Preferably, the copper sheet is very thin to later enhance bonding so that completed devices can later endure temperature cycling tests.
High conductivity copper foil of very high quality is generally used to make such tape. The foil is typically 0.0178 m.m. to 0.071 m.m. in thickness and is annealled to obtain "dead soft" ductility. Foil of this type is very delicate and requires careful handling. Without such care, it will become wrinkled, warped or otherwise distorted as it is wound upon reels or indexed for device assembly.
The leads formed in the copper foil are typically arranged in clusters with 8 to 32 finger-like leads per cluster. Typical dimensions are such that a tape may yield in the range of 64 clusters per lineal foot for bonding chips thereto.
Each lead should be accurately placed to within about 0.0025 m.m. of its desired position and each edge must be sharp and carefully delineated. Otherwise, registration of leads-to-bonding pads or of leads-to-leads will be affected during bonding operations. Registration is also affected by the way indexing is accomplished. Therefore, close tolerance perforations typically are provided in the tape to accommodate sprocket teeth used to engage and index the tape.
The fine and precise linework required to form such a tape is conventionally accomplished reliably and reproducibly using photolithographic techniques. A master mask is prepared with the utmost precision, typically using a laser machine system controlled by a computer. The mask is used repeatedly to obtain exact line location, sharp and clean line definition and excellent reproducibility of images formed by the mask.
The copper strip is first cleaned and then coated with a light-sensitive coating, such coating being normally resistant to etchants. The coated strip is then positioned into a spaced relationship with the master mask through which the coating is selectively exposed to intense ultraviolet light to form a latent image of a desired pattern in the etchant-resistant coating. Then the pattern is formed into the strip without downgrading the quality of the foil, without destroying its dimensional integrity and without degrading the precision of the linework in the pattern.
Forming the pattern in the strip generally includes the steps of developing the latent image of the pattern in the coating, cleaning the image to make it clearly patent and etching away unwanted areas of copper foil not protected by the etchant-resistant outline of the image in the coating to complete the pattern. Economics of manufacture dictate that these forming steps be performed substantially simultaneously as the strip is driven through a succession of such forming steps.
Prior art methods of driving a strip such as that disclosed in U.S. Pat. No. 4,013,498 to J. J. Frantzen et al. employ spools, reels and guides which apply pressure to the entire width of surface both above and below the strip. Generally the strip is freely suspended as it passes through solution spray chambers and the spray impinges upon the strip from the top or bottom of the chamber. When such methods are used to grip and drive the strip described herein a risk is taken because the gripping pressures tend to damage the coating and distort or otherwise degrade the latent image therein. Furthermore, the tension required to suspend the strip sufficient to withstand the spray impingement forces is not reliably endured by the low strength of the delicate copper foil.
It is, therefore, desirable to have methods and apparatus to grip and drive the strip without disturbing the latent image of the pattern in the strip. Furthermore, it is desirable to form the pattern in the coating on the strip without impinging solution sprays upon suspended portions of the strip.
The developing, cleaning and etching steps also pose many problems when tried in a successive and continuous manner on the constantly moving strip such as the strip described herein. Each spray solution used in these steps contains different chemicals having reaction time intervals which are also usually different. Further, it is desirable to conserve these solutions by recirculating and reusing them. Yet such reuse of solution causes reaction chemicals to be depleted and impurities to build up in the solution. Thus, the rates of chemical reaction are affected by such reuse of solution.
It is, therefore, desirable in the successive and continuous forming steps to coordinate and adjust the time intervals during which the pattern is exposed to the solution sprays at each step. It is also desirable to adjust each time interval from time-to-time as each solution becomes depleted of its chemicals or contaminated with impurities. Furthermore, it is desirable to make these adjustments without expensive electrical or mechanical controls such as spray valves, timers, switches or variable-speed drive mechanisms.